1. Field of the Invention
The present invention relates to a photosensor for detecting light, a target detection method of the photosensor, and a display panel equipped with the photosensor.
2. Description of the Related Art
As disclosed in, for example, Jpn. Pat. Appln. KOKAI Publication No. 6-236980, there has hitherto been known a photosensor constituted of a plurality of adjacently arranged light receiving elements formed from amorphous silicon (hereinafter referred to as a-Si) as thin-film transistors (hereinafter referred to as a-Si TFTs), which produce electrical energy corresponding to irradiated light.
FIG. 7 is a diagram showing one example of the light-electricity characteristics of an a-Si TFT used in such a photosensor (Ids [A] is measured when the illumination of irradiated light is used as a parameter under conditions such that TFT size (W/L)=180000/9 μm, and terminal voltages Vs=0 V and Vd=10 V). FIG. 7 shows that the drain-source current Ids increases with illumination. The Ids prominently increases especially in a reverse bias region (Vgs<0), and the photosensor generally uses the characteristic of this region to detect the illumination of the irradiated light as the change of the Ids.
Such a photosensor (light detector) is used to detect the presence of, for example, a pachinko ball (pinball) or paper as a detection target (e.g., Jpn. Pat. Appln. KOKAI Publication No. 2002-148353).
However, in such an application, there is a high possibility of disturbance light entering the light receiving element in addition to light radiated from a light source and reflected by the detection target. If the disturbance light enters the light receiving element, an erroneous operation may be caused due to decreased accuracy in the detection of the detection target.
Thus, Jpn. Pat. Appln. KOKAI Publication No. 2002-148353 has proposed a detection method in which the light emission timing of a light emitting element for emitting light to be applied to a detection target is compared with the light receiving timing of a light receiving element, and in which it is recognized whether light reflected from the detection target or outside light has been received in accordance with whether these timings are synchronized with each other.
However, the detection method disclosed in Jpn. Pat. Appln. KOKAI Publication No. 2002-148353 is not capable of providing an inexpensive photosensor free of erroneous operations due to a complicated detection circuit, the necessity of strict timing control, etc.